This invention relates generally to medical devices and, in particular, to a dilator sheath using electrical energy to separate encapsulating tissue from an implanted cardiac electrical lead.
While cardiac electrical leads typically have a useful life of many years, over time pacemaker and defibrillator leads fail. Unfortunately, by the time they fail, they have become encapsulated by fibrotic tissue against the heart itself or the wall of the vein. Encapsulation is especially encountered in areas where a device has caused tissue injury. Encapsulation is the body""s healing response to protect surrounding tissue from further injury. Scar tissue may also form due to continual device-related mechanical stresses (i.e., excessive pressure), infection, or inadequate blood supply to the site. The fibrotic tissue is tough and makes it difficult to remove the lead from the patient without causing trauma to the heart or great vessels. For example, when small diameter veins through which a pacemaker lead passes become occluded with fibrotic tissue, separating the lead from the vein can cause severe damage to the vein such as dissection or perforation.
To avoid this and other possible complications, some useless cardiac leads are simply left in the patient when the pacemaker or defibrillator is removed or replaced. However, such a practice can incur the risk of an undetected lead thrombosis or pulmonary embolism. Such a practice can also impair heart function, as multiple leads can restrict the heart valves through which they pass. Furthermore, such a lead can later become infected.
There are, of course, many other reasons why removal of a useless lead is desirable. For example, if there are too many leads positioned in a vein, the vein can become totally occluded. Multiple leads can be incompatible with one another, interfering with the pacing or defibrillating function. An inoperative lead can migrate during introduction of another adjacent lead, and mechanically induce ventricular arrhythmia. Some recalled leads include J-shaped retention wires that have been known to fracture and protrude through the insulation, causing several reported deaths. Other potentially life-threatening complications can require the removal of the lead as well. For example, removal of an infected pacemaker lead is considered mandatory in the presence of septicemia and endocarditis. Other necessary indications such as pocket infection, chronic draining sinus, and erosion can lead to significant morbidity if the lead is not removed.
Until recently, manual (or direct) traction, weighted (or sustained) traction, and open-heart surgery/thoracotomy have been the most common methods of removing useless or infected cardiac leads. Manual and weighted traction involve the risk of tearing the myocardium and are largely ineffective for leads extensively encased in fibrotic tissue. This procedure is also ineffective in patients with multiple leads when these leads become scarred together at common fibrous binding sites. The risks and trauma associated with an open surgical approach are obvious. Yet another method of transvenously extracting a cardiac lead is by the use of a grasping device, such as a forceps or basket that is positionable around the outer surface of a lead or fragments of a lead. The use of forceps or a basket for lead withdrawal is complicated by the fact that the lead should first be freed from any encapsulating material surrounding it along its path. Furthermore, tearing of the myocardium or vessels can result during attempted extraction. Many of these problems were overcome by the development of a system of tools and methods for transvenous extraction of pacemaker leads and other elongated objects such as catheters. Many of these tools and procedures were developed with the assistance of Cook Pacemaker Corp., Leechburg, Pa., as evidenced by U.S. Pat. Nos. 4,988,347; 5,013,310; 5,011,482; 4,943,289; 5,207,683; 5,507,751; 5,632,749; and corresponding foreign patents. The preferred method involves positioning a lead removal tool or xe2x80x9clocking styletxe2x80x9d inside the coiled wire of the lead to engage the coil. Once the locking stylet is positioned inside the coil, reinforcement is provided and extraction forces are concentrated at the lead tip. By using a sheath to apply countertraction at the embedded tip as the lead is extracted, damage to the myocardium can be largely avoided.
Typically, the locking stylet alone does not provide the tensional force required to safely extract the lead due to excessive fibrotic or scar tissue that has encapsulated the lead against the vessel or myocardial wall. Dilator sheaths formed from plastic or metal tubes can be used to disrupt and separate the encapsulating tissue. Commonly, two coaxial dilator sheaths are positioned over the lead and advanced therealong for loosening the lead from the fibrotic tissue on the vein wall. Plastic sheaths are flexible for bending around the natural anatomical curvatures of the vascular system. A problem with the plastic dilator sheaths is that the leading edge of the dilator sheath is weak and can lose its edge and buckle onto the lead during use. As a result, the plastic dilator sheath can become damaged and unusable before the lead is loosened from the fibrotic tissue. Furthermore, the tips of the flexible plastic sheaths can deform when subjected to tough fibrotic tissue. This problem is further heightened when the sheath is bent around a vessel curve. Metal dilator sheaths provide a sharp leading edge for encountering fibrotic tissue. A problem with some metallic dilator sheaths is that they are relatively inflexible and resist bending around natural anatomical curvatures. As a result, a metallic dilator sheath can be difficult or impossible to advance toward the distal end of the pacemaker lead without injuring or obliterating the vein. Flexible metallic dilator sheaths have been developed to address the problems associated with plastic sheaths and rigid metal sheaths. While very effective for their intended use, even metal sheaths are inadequate for the toughest fibrotic tissue and calcification in a vessel. The tensile strength of the fibrous tissue increases with time. Eventually the tissue can even differentiate into cartilage or bone. Attempted separation of difficult fibrotic tissue can cause mechanical trauma to the vessel. Data show that 5.4% of all attempted lead extractions are not successful and 7.5% are only partially successful, almost entirely due to the presence of excessive scar tissue. Lead fragility is another problem and generally escalates over time when a lead has a design flaw or has been structurally compromised.
U.S. Pat. No. 5,423,806 of Dale et al. discloses a laser catheter for ablating encapsulating tissue during the extraction of pacemaker leads. Using directed high energy to burn, desiccate, or melt the tissue encapsulating the lead can reduce the length of the procedure and increase the number of leads that can be extracted. In the practiced embodiment of U.S. Pat. No. 5,423,806, optical fibers are arranged circumferentially around an open lumen through which the lead passes. One problem with this embodiment is that tissue can be readily cored and plug the internal lumen of the device, thus making forward or reverse movement of the device extremely difficult. The laser device is used in combination with a plastic outer sheath and tracks over the lead as the distal tip of the laser burns through any obstructive tissue surrounding the lead. Partly due to the difficulty in visualizing the treatment site, a significant disadvantage of this approach is the risk of burning though the vessel wall or myocardium. This is especially a problem if sufficient tension is not constantly maintained on the lead during the procedure, allowing the distal tip of the laser to angle toward the wall of the vessel or myocardium. This could pose an unacceptable risk for the large number of lead extractions that are elective procedures and do not involve life-threatening indications.
Alternative embodiments of the laser catheter suggested by the Dale reference include having the optical fibers grouped on one side of the catheter or utilizing a single fiber. Either would permit more precise ablation of scar tissue surrounding the lead if the point of ablation can be manipulated and selectively rotated away from the vessel wall. It is suggested that a stylet could be inserted into an additional lumen of the catheter to facilitate rotational control. While providing the physician with control over the point of ablation during the procedure should reduce the risk of accidentally penetrating the vessel wall, the effectiveness of the laser catheter is still limited by the fragility of the optical fibers. Given the tendency of optical fibers to break when subject to lateral bending or rotational forces, current laser catheter designs are not particularly torqueable. An annular arrangement of optical fibers, with its disadvantages, is used that does not require that the catheter be rotated. However, even when merely navigating a laser catheter through a tortuous angle, breakage can occur that can result in the catheter burning through itself or the cardiac lead insulation due to the large amount of heat generated. These disadvantages, along with the much higher cost, limit the laser catheter as an alternative to manual sheaths.
The foregoing problems are solved and a technical advance is achieved by a medical device for separating an elongated structure such as an electrical cardiac lead implanted in biological tissue. The medical device comprises an inner elongated dilator sheath having a distal end and a passage extending longitudinally therethrough. The medical device further comprises an electrical conductor positioned about the distal end and passage of the inner elongated sheath. The passage of the sheath is sized and configured for placement of an elongated structure, such as an electrical cardiac lead, implanted in biological tissue, such as a vessel leading to or from the heart. When energized, the electrical conductor electrically separates or ablates biological tissue from the elongated structure implanted therein and placed in the passage of the inner dilator sheath. Advantageously, the distal end of the inner elongated dilator sheath is at least partially beveled for mechanically loosening and separating encapsulated tissue from the elongated electrical lead. As a result, the electrical conductor and the mechanical configuration of the inner dilator sheath work in concert with each other to provide separation of extremely tough encapsulating tissue and stubborn calcification deposits from the elongated electrical structure. In addition, the sheath can disrupt the fibrous tissue bands which commonly bind multiple cardiac leads together. The beveled distal end also includes a transverse face that advantageously positions the electrodes of the electrical conductor (therein) so as to establish and maintain an electrical, tissue ablating arc therebetween. The tissue ablating arc also advantageously maintains a necessary gap between the obstructive tissue and the end of electrodes as the dilator sheath is eased forward.
The radio frequency dilator sheath further includes an outer dilator sheath, which also advantageously has a beveled distal end that is coaxially positioned over the inner dilator sheath for providing coordinated longitudinal and rotational movement with the inner dilator sheath for separating encapsulating tissue from an implanted lead.
In the preferred embodiment, first and second electrical conductors are advantageously positioned in the wall of the inner dilator sheath and about the distal end thereof. When connected to a source of radio frequency energy, an electrical arc of radio frequency energy is selectively established between the conductors for heating, cutting, ablating, or melting encapsulating tissue and calcification deposits away from the implanted lead. The electrical conductors preferably have a tungsten electrode tip so as to prevent deterioration of the conductor with an electrical arc emanating therefrom. The electrode tip is conveniently connected via a connector sleeve to a supply conductor which exits the inner dilator sheath about the proximal end thereof.
In another illustrative embodiment, the electrical conductor or conductors are positioned in longitudinal recesses formed in the outer surface of the inner dilator sheath about the distal end thereof. The electrode tip is positioned in the recess and fixedly positioned therein with a biocompatible material, such as a medical grade adhesive or epoxy. An outer wrap, such as a shrink-wrap tube, is positioned around the inner dilator sheath as well as the electrical conductors to fixedly position and mechanically support the remaining portion of the electrical conductors along the remaining length of the inner dilator sheath. As previously suggested, an outer coaxial dilator sheath is also used in combination with this alternative embodiment for separating encapsulating tissue from an implanted elongated structure.
In yet another embodiment of the radio frequency dilator sheath, a plurality of electrical conductors, for example, three, are positioned in the wall of the inner dilator sheath and are selectively energized in pairs or simultaneously to provide further circumferential electrical separation of encapsulated tissue from the implanted lead positioned in the main passage of the dilator sheath.
The inner and outer coaxial dilator sheaths of the present invention each preferably comprises an elongated tubular member of a biocompatible material, the inner sheath having a high temperature resistance or high continuous use temperature preferably over 500xc2x0 F. In the preferred embodiment, the outer coaxial dilator sheath comprises a polypropylene material, whereas the inner dilator sheath comprises a radiopaque polytetrafluoroethylene material. By way of example, the radiopaque material can include bismuth, barium, bismuth carbonate, platinum, tungsten, or any other commercially available radiopaque material. Other high-temperature resistant biocompatible materials having a heat deflection temperature of, for example, 500xc2x0 F., include fluorinated ethylene propylene, polyetheretherketone, polyetherimide, polyphenylsulfone, and polyimides.
Preferably, the electrical conductors of the radio frequency dilator sheath include a high temperature electrode tip of a material such as tungsten so as to advantageously prevent deterioration of the conductor due to the electrical arc emanating therefrom during separation of tissue from the implanted structure.
One or more conductors can extend over or in the distal end of the inner sheath. Each conductor can be located on the outer surface, or in a recess on the outer surface, or in a passageway at the distal end region of the inner sheath.